1. Field of the Invention
The present invention relates to a self-startup circuit for a direct current (DC) fan, and more particularly to a self-startup circuit which keeps running of a DC fan when no pulse signal is inputted.
2. General Background
Developments in today's highly information-intensive society have led to remarkable improvements in performances of electronic devices. During operation of many contemporary electronic devices such as central processing units (CPUs), large amounts of heat are produced. Typically, a fan is used to facilitate removal of heat. The fan must be running stably, so as to prevent the device from becoming unstable or being damaged. If the fan runs unstably or even ceases running, heat generated from the CPU will not be dissipated on time and will ruin the CPU. A startup circuit of the fan is quite important to ensure normal running of the fan.
FIG. 6 shows a conventional startup circuit of a DC fan. A Pulse-Width Modulation (PWM) control signal from a control chip directly drives transistors Q70 and Q100. The transistor Q70 and Q100 directly drives a DC fan. A high capacity electrolytic capacitor C63 is provided for wave filtering. When the startup circuit is started, a large start-up current is generated in the start-up instant, which will give an impact to or even ruin the transistors Q70 and Q100. Although the high capacity electrolytic capacitor C63 is adopted, voltage ripples still exist which makes the fan to rotate unstably. When the PWM control signal is very low and close to zero, that is, the duty cycle of the PWM control signal is very low and close to zero, the transistor Q71 is turned off. The fan therefore has incipient fault of stop running, thus cannot protect the CPU efficiently.
What is needed is a self-startup circuit which keeps running of a DC fan when no pulse signal is inputted.